Thermoelectric heat pump



July 21, 1959 R. w. FRITTs Erm. 2,896,005

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United States Patent 2,896,005l THERMoELECrR'IC HEAT PUMP lRobert W. Fritts, Elm' Grove, Wis., and Sebastian Karrer, Port Republic, Md., assignors,*by mesne assignments, to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware f Application June 1, 1955,'seria1N0. I512,436 v 13 Claims. (c1. y13s- 5) forrnance factors.

Another object is to provide thermoelectricl heat pumps having at least one improved thermoelectric element which when connected in circuit with a source of direct current altords heat transfer through the thermoelectric element with resultant cooling of one portion and heating of another portion of the element in sudicient amount for practical use.

Another object is to provide heat pumps of the character aforedescribed comprising thermoelectric elements 2,896,05 Patented July 21, 1959 through the junction. The proportionality factor between heat absorption or evolution rate and electric current is called the Peltier coeicient and is equal to the heat transferred when unit quantity of electricity traverses the junction. The Peltier coeflicient may be directly determined as the product of absolute temperature and the derivative with respect to temperature of the Seebeck Accompanying such eiect under the circumstance stated is a transformation between thermal energy and electrical energy which takes place within each individual conductor, called the Thomson effect, which is delined as the heat absorbed or emitted per unit volume by a unit current passing through a unit temperature gradient. Such effects are reversible.

Metallic conductors, when joined to form a thermoelectric junction as aforedescribed, exhibit low Peltier coefficients and low thermoelectric power. Moreover, such metallic conductors exhibit high thermal and electrical conductivity.

When an external voltage is introduced into the ther- 1 moelectric circuit, as aforementioned, the electric charges mo've according to the laws of conduction and as they pass the junctions of dissimilar conductors absorb or emit heat, thereby suiering a change in their electrostatic potential. This is called Peltier heat pumping, the Thomson elect being neglected since the temperature gradients within the thermoelectric elements are quite low in pumping.

In addition to the aforementioned reversible thermoelectric eiects there are two other more common eiects which must be considered in understanding the'behavior of semimetallic compositions and more particularly certain binary and ternary compositions having semiconductor-like conductivity or conductance.

Other objects and advantages will hereinafter appear or will become apparent to those skilled in the art with reading of the following specification.

In the drawings:

Figure 1 is a somewhat schematic illustration of a single thermoelectric element heat pump embodying, the invention;

Figure 2 is a `somewhat schematic illustration of a compound or double thermoelectric element heat pump comprising thermoelectric elements of opposite polarity embodying the invention;

Figure 3 is a graphic illustration of certain leadselenium-tellurium compositions comprising thermoelecltric heat pump elements of the invention;

Figure 4 is a graphic illustration of certain leadselenium-sulphur compositions comprising thermoelectric heat pump elements of the invention;

Figure 5 is a graphic illustration of certain electrical characteristics of negative thermoelectric heat pump elements embodying the invention;

Figure 5A is a graphic illustration of certain electrical characteristics of certain positive thermoelectric heat pump elements embodying the invention;

Figure 6 is a graphic illustration of the performance factors of heat pumps embodying the invention; and

Figure 7 is a graphic illustration of certain electrical characteristics of additional positive thermoelectric heat pump elements embodying the invention.

When a direct current is passed through a junction between two dissimilar conductors the junction is heated or cooled depending not only upon the direction of current flow but also upon the nature of the conductors. This heating or cooling of the junctionsis called the Peltier eiect and is linearly dependent upon the current of a thermoelectric heat pump. These are the conventional Joule heating, or 12R dissipation (wherein I denotes current and R denotes resistance), and normal heat conductivity. In most applications of the Peltier heat pump these effects, which are irreversible, should be minimized for they are detrimental. Therefore, the resistivity and thermal conductivity of thermoelectric elements used in `heat pumps should be as low as possible.

.- In Figure l the reversible Peltier eitect is illustrated for a single thermoelectric element 10 conductively joined as at A and B to ordinary metallic conductors 11 and 12 completing a circuit to a source of direct current. The conductivity type of the thermoelectric element 16 is s uch that as unit current ows across junctions A and B, Pa units of heat are absorbed at junction A and Pb units of heat are emitted at B. The magnitudes of Pa and Pb depend upon the value of Peltier coeicient between the therrnoelectric element 10 and the conductors 11 and 12 and upon Vthe temperatures of the respective junctions. Where the junction temperatures are identical the heat absorbed at A equals the heat emitted at B.

If the initial temperature distribution in the pumping element is constant at To, then with time the absorption of heat by the current at A gradually lowers the junction temperature, Ta, and the emission at B gradually raises the temperature, Tb.

The extent to which Ta and Tb dier from Tn depends upon the magnitudes of the irreversible effects aforementioned as well as upon the magnitude of Peltier The electrical energy input to the pumping element in the form of 12R dissipation within the volume of the thermoelectric element raises the temperature of the thermoelectric element slightly. This irreversible input alters the temperature distribution within the thermoelectric element until it is balanced by an equal flow of heat out through the junctions. The other irreversible effect, heat conduction, causes a ow of heat from the high temperature region' to the cooler portions; this is denominated as Qc.

The establishment of temperature differences lbetween the junctions and their external Venvironments induces heat flows therewith according to the laws of heat conduction; these are specified as Qa and Qb. When the current fiow is held constant and the temperature distribution assumes a steady state then the total ow of heat into junction A equals the rate of Peltier absorption at 'u junction A. (PnzQa-j-Q-l-l/zlzR). Similarly, the flow of heat from junction B equals the rate of Peltier emission at junction B (Pb=QblQc12R/2) As will be apparent, the important parameters for the operation of the pump are Qa, Qb, Ta and Tb. Q,l specifies the rate at which heat can be extracted from an external source in contact with the cooling junction A. Qb specifies the rate at which heat can be delivered (exhausted) to an external source in contact with the heating junction B. T b-Ta is the temperature difference through which the thermal energy is transferred. These parameters will be used henceforth in describing the performance of pumps; the other entities relating to phenomena occurring within the pumping element were specied merely to explain the pumping action and will not be mentioned further; It should be noted, however, that the difference between Qa and Qb is merely the electric power required to operate the pumping cycle. (Qb-Qa=EI- I2 Rl-Ide). In the last, "de is the differential thermal arising from the induced temperature difference Tlf-Ta.

The direction of heat transfer of the thermoelectric element of Figure l is characteristic of elements having positive polarity of thermoelectric power and conductivity and with the current flowing in the direction indicated. For thermoelectric elements of negative polarity the heating and cooling of the junctions are just reversed. This is illustrated in Figure 2 wherein is shown a thermoelectric element 13 of positive polarity and a thermoelectric element 14 of negative polarity arranged in series circuit with respect to each other and a source of direct current. The arrow indicates the direction of electric current in the conventional sense-the electron ow is, of course, in the opposite direction. Accordingly, heating occurs at the junctions indicated as C and D between the thermoelectric elements 14 and 13 and electrical conductors 15 and 16, respectively, while cooling occurs at junctions designated E and F between thermoelectric elements 14 and 13 and a metallic conductor 17, respectively. In the arrangement shown in Figure 2, the heat transfer and consequent heating of junctions C and D and cooling of junctions E and F are additive, as will be apparent, thereby affording a heat pump of greater efficiency than one comprisinga single thermoelectric element as shown in Figure l. The heat pump of Figure 2,is, of course, possible only by virtue of utilization of thermoelectric elements 13 and 14 exhibiting Peltier and conductivity opposite in sign. The conductors, 15, 16 and 17 are metallic incomposition and hence exhibit electrical and thermalproperties characteristic of metals as aforedescribed.

From the foregoing parameters we have derived an expression affording a convenient measure for comparison of thermoelectric elements and evaluation of their utility in heat pumps, which is hereinafter denominated the Performance Factor of the heat pump. This expression is rrz/ZKp wherein vl-:Peltier coefficient K=Thermal conductivity p=Electrical resistivity This expression is derived directly from an analysis of heat flow in thermoelectric heat pumps. The parameters involved are determined by conventional techniques and the term above (Performance Factor) is a measure of the capacity of the heat pump to create a temperature difference and convect heat through this difference.

The absorption or evolution of heat at a junction, as aforedescribed, is directly proportional to 1r. The irreversible flow of heat down the temperature difference is proportional to K andthe Joule (12R) dissipation is proportional to p. The values of 1r and p are related through the nature of the conduction processes in the compositions hereinafter described. From the foregoing expression it will be observed that the best heat pump will comprise thermoelectric elements having at one and the same time high values of Peltier (1r) and low values of thermal conduction (K) and of resistivity (p), or high conductivity.

The performance of thermoelectric heat pumps ernbodying this invention may also be stated in terms of the heating and cooling afforded at the junctions in relation to the amount of electrical energy required for such heating or cooling. An expression of this factor is denominated herein the Performance Coefficient. The performance coeicient of a heat pump (with respect to the cooling junction) is definedA as the ratio of heat absorbed at the cooling junction to the electrical energy dissipation within the pump and can be set forth as follows:

The performance coeicientof a heat p ump (with respect `to theV heating junction) is defined as the ratiojof the heat exhausted at theheating junction to the electrical energy dissipation within the purnp and can be setmforth as f9l19WS= 9 6: Q. E] nQl .-Qa The performance coefficient of heating is equal to the performance coeflicient fof cooling plus one.

To provide heat pumps of high rate of heat transfer and high `efficiency as aforementioned comprising one or more-thermoelectric elements (as illustrated respectively in Figures-land 2), we have produced thermoelectric elements of Vnew-compositions exhibiting electrical characteristics as aforementionedfsuperior forheat pumping purposes` than.. known metallic alloys `and therrnoelectric elements of known semiconductive compositions.

tWe A,have found that` certain alloys or intermetallic compounds exhibit very favorablerelationships between Peltiercoeicient and resistivity and simultaneously favorable -valueswof Athermal conductivity. Further, we have found that the values of-Peltier coeiiicient and resistivity may `be arbitrarily and advantageously altered to afford a-wide range ofperformance` factors without significant changes in thermal conductivity. Metals and metalic alloys exhibit low values of Peltier coeicient and resistivity andsimultaneously high values of thermal conductivity. On the other hand, semiconductors exhibit low values of thermal conductivity and high values of Peltier coefficient and resistivity. Performance factors for metals .arelow because of the Ahigh values of thermal conductivity. Performance factors for semiconductors are low because of their high resistivities. Our invention i is Vbasedon the fact that we havefound a way to alter the electrical properties of certain intermetallic compounds (which in a pure state exhibit high Peltier coefficient and resistivity and low values of thermal conductivity) to reduce their resistivity without a proportionate reduction in Peltier coefficient or a significant increase in thermal conductivity. These `intermetallic compounds, which in a pure state are semiconductors, 4are altered by adding small concentration ofl beneficial impurity which render the semiconductors somewhat more metallic in nature, thereby vastly improving the performance factor. The resulting compositions exhibit high values of Peltier coeiiicient andlow values of resistivity and thermal conductivity in a relationship which 3cannot be achieved in eitherpure` metals or pure semiconductors. Such resulting compositions we `have `denominated semi-metallic alloys o r compositions (to distinguish them yfrom metallic alloys on the one hand and semiconductor alloys on the other-hand). VSuch semi-metallic alloys or compositions for `7the rst time afford Peltier heatpumping with induced ternperaturedifferences and `pumping rates of suicient magnitude Lfor `utilization in practical applications.

. plete achievement.

The semi-metallic alloys or compositions aforemenizionedv may, we have found, be characterized as binary metallic compounds'of slightly imperfect composition, i.e. containing beneficial impurities constituting departures from perfect stoichiometry by reason of an excess of one of the metals over the other, .and/or containing added beneficial kimpurity substancesl denominated hereinafter promoters Such semi-metallic compositions have semiconductor-like conductance (both electrical and thermal, as aforementioned). Semi-metallic alloys or compositions also include mixtures of such binary metallic compounds, which' may be denominated ternary metallic alloys or compositions. l

More specically, we have found especially useful for heat pumping thermoelectric elements comprising lead and at least one member of the group tellurium, selenium and sulphur in proportions hereinafter described and to which may be added beneficial impurity substances in the form of promoters, as Will also hereinafter be described. As will appear, certain of these alloys or compositions exhibit. negative and certain exhibit positive electrical characteristics thereby affording, if desired, a compound or double. heat pump unit as shown in Figure 2.

Referring now to Figure 3, there are graphically illustrated therein a multiplicity Yof examples of semi-'metallic compositions or alloys for thermoelectric heat pump elements comprising leadand tellurium or selenium. It will be observed that the horizontal coordinate of this graph represents the various proportions of tellurium and selenium given in atomic percent and ranging linearly from tellurium containing but a trace (as hereinafter dened) of selenium on the left to selenium containing but a trace of tellurium on the right. The left-hand vertical scale (in terms of percent by weight) gives the amount .of lead which can be alloyed with the tellurium, selenium or tellurium-selenium constituent for any proportions, of the latter While the right-hand vertical scale conversely gives the percent by weight of the tellurium, selenium or selenium-tellurium constituent for any proportions of the latter in the final composition, the remainder, of course, being lead.

Figure 3 ygraphically illustrates compositions or alloys comprising lead and either tellurium or selenium or both since, we have discovered, selenium and tellurium when alloyed with leadwithin the proportions indicated are mutually soluble throughout the range of compositions illustrated and that tellurium and selenium are interchangeable for purposes of providing suitable thermoelectric heat pump elements falling within the class of binary metallic compounds aforementioned, and due to such mutual solubility, such binary compounds of the constituents indicated may bermixed, as is also graphically illustrated, toprovide'the ternary alloys or compositions .also hereinbeforeV mentioned. The discovery of such --i'nterchangeability of selenium and'tellurium is important -in tlie'economic manufacture of thermoelectric elements .of the character aforeindicated inpthat it eliminates the .necessity for separating selenium' from tellurium and "vice versa (these two constituents being invariably found togetheras contaminants one to the other), a dificult :and expensive procedure, and indeed impossible of com- Accordingly, even the end extremes of the compositions illustratedin Figure 3, i.e. that of '.the'left-hand end of the scale (35.0% to 38.05% by weight tellurium, remainder substantially all lead) may be considered as containing at least a trace of selenium, while the composition at the other end of the scale (25.0% to 27.55% by weight selenium, remainder substantially all lead) may be considered as'v containing at least a trace of tellurium. Accordingly, where the term trace is used hereinafter in the specification and claims, it is to be understood as meaning amounts of the specified constituents and/or contaminants so small as to Vdefy detection but which must be assumed to be present due to the. impossibility ofachieving absolute purity.

- Again referring to Figure 3, by way'of example, it will be observed that a thermoelectric element of lead, selenium and tellurium aiording the desired characteristics could consist, as aforedescribed, of a selenium'- tellurium constituent in which the selenium is but a trace. In this case such constituent should'constitute from 35.0% to 38.05% by weight of the composition, the balance (65.0% to 61.95% by weight) being lead. On the other extreme, where the selenium-tellurium .constituent consists almost entirely of selenium, with but a trace of tellurium, such constituent should comprise from 25.0% to 27.55% by weight of the final composition, the remainder (from 75.0% to 72.45% by weight) being lead. These extreme compositions, aforedescribed, may for purposes of this specification and claims be denominated terminal compositions and maybe considered .binary metallic compounds of slightly imperfect compositions as will hereinafter appear.

As a further example, Where the selenium and tellurium are equal (in atomic percent) in the ,selenium-tellurium constituent, the latter should constitute from 30.0% to 32.8% by weight ofthe composition, the remainder (70.0% to 67.2% by weight) being lead. Such a composition, as well `as all other compositions illustrated inrFigure 3, intermediate the terminal composi- -tions aforedescribed, are mixtures (in various proportions) of the terminal compositions and since they contain both selenium and tellurium aswell as llead, are ternary alloys or compositions.

Referring now to Figure 4 there -are graphically illustrated therein a further multiplicity of examples of semi-metallic compositions or alloys for thermoelectric heat pump elements comprising lead and selenium or sulphur. As is the case in Figure 3, the horizontal coordinate of the graph of Figure 4 represents the various proportions of selenium and sulphur given in atomic percent and ranging linearly from selenium containing but a trace, as above'deiined, of sulphur on the left to sulphur containing but a trace of selenium on the right. The left-hand vertical scale (in terms of percent by weight) gives the amount of lead which can be alloyed withV the selenium, sulphur of selenium-sulphur constituent for any proportions of the latter, While the righthand vertical scale conversely gives the percent by weight of the sulphur, selenium, or selenium-sulphur constituent for any proportions of the latter in the final composition, the remainder, of course, being lead.

Figure 4 further graphically illustrates compositions or alloys comprising lead and either selenium` or sulphur or both since, we have discovered, selenium and sulphur when alloyed lwith lead Within the proportions indicated are mutually soluble throughout the range of compositions illustrated and that selenium `and sulphurY are interchangeable for purposes of providing suitable thermoelectric heat pump elements falling within the class of binary metallic compounds aforementioned, and due to such mutual solubility such binary compounds of the constituents indicated may -be mixed as is also illustrated to provide the ternary alloys or compositions also hereinbefore mentioned. The discovery of such interchangeability of selenium and sulphur is important fin the economic manufacture of thermoelectric elements of the character aforeindicated in that-it eliminates the necessity'for separating selenium from sulphur and vice versa (these two constituents being invariablyV found together as contaminants one to the other), a difficult and expensive procedure and indeed impossible of achievement. Accordingly, even the end extremes of the compositions illustrated in Figure 4, i.e. that of the left-hand end of the scale (25.0% to 27.55% by weight' selenium, remainder lead) may be considered as containing at least a trace of sulphur, While the composition at the other end of the scale (12.80% to 13.37% by weight sulphur, remainder substantially all lead) maybe considered as containing atv` least 'a trace of selenium.

, Again referring 'to' Figure y4,r .by way of example, it will be observed ,that a "thermoelectric element of lead, f

sfeleniumand sulphur affording the desired characteristics could consist, as gaforedescrihed, of' a selenium-sulphur i constituent inwhich the sulphur is rbut a trace. 'In this case, such constituent 'should constitute from 25.0% to 27.55% Vby 'Weight'of the composition, the balance almosty entirely yof sulphur with but a trace of selenium,

where the formed composition is putthrough a recryfs-r tallization step,y to jbe hereinaftery described, lto provide the i iinal or `end composition 'of the order of purityr afore-Q indicated. Accordingly,y the starting ingredients or, in

any event, the yinal'composition where mentioned in this` kspecification and appended claims are tor bennderstood (75.0% to k72.45% by Weight) being lead. On the rother cxtreme,'where the selenium-sulphur constituent consists f hence `rrnistbe substantiallyremovedby purification. For y f example, copper is one example of such an impurityrhavy the desiredfelectrical .propertiesk aforementioned may be yAs ya yfurther example, Where'the selenium and sulphur arey equal (in atomic percent) in yther s'elenium-sulphury y constituent, the latter should constitute from 18.90% toy 20.46%y 'by' weight of. the composition, the remainder (81.10%to `79.54% 'by weight) beingk lead.y rSuch a composition, as ywell as other compositions illustratedk `in y 'Figure' 4`intermediate ,the terminal compositions afore-y `described',fare mixtures (in various proportions) yofy the,

terminal compositions and since theycontainboth seleuiy rkum 'and sulphur aswell `asy ilead, `are ternaryy alloys or compositions. f f

We have also discovered that kthe terminal `composi- 'tionsy aforedescrib'ed consisting substantially yentirely of 'lead and tellurium may yto a limited extent contain somer sulphur (ic.y to the extent thatk sulphur is yfound yasy a contaminant incommercially available tellurium) as well as selenium. Similarly, theterminal compositions afnrey described consisting substantially' of lead and selenium may contain to' a limitedy extent ytellurium and sulphur,

as they case may be, and the terminali compositions rafore-y n described consisting substantially yof lead and sulphur may contain telluriurn to aflimited extent as yaforeindicated as well as selenium.y yLikewisey anyfof' the intermediatek or ternary alloys or compositions consisting of lead and at least two members of the group tellurium, selenium, sulphur, may contain such limited amounts of the other element of the group.

The proportions and ranges of the various constituents aforementioned as illustrated in 4the graphic representation constituting `Figures 3 and 4 of the drawings must be considered critical if the compositions illustrated are to have the electrical properties desired in heat pumps as aforementioned. The minimum limits of the lead constituent in the compositions of the invention (illustrated graphically by the lower curve in Figures 3 and 4) must be regarded as critical since if the lead content is significantly less than this amount for any particular proportions of the constituents, desired values of Peltier and resistivity 4will not be afforded and the significant electrical and mechanical properties will not be reproducible. On the other hand, if the lead content of any composition appreciably exceeds the maximum limit (illustrated graphically by the upper curve of Figures 3 and 4) the resulting composition, we have found, is too metallic in nature to afford electrical characteristics satisfying the objects of this invention.

u Not only are the proportions and ranges aforedescribed considered to be critical, but so also is the purity. More 'speciiically, the limit of tolerable metallic impurity in the final composition has been found to be on the order of 0.01% and the composition must be substantially oxygen free if the mechanical and electrical properties desired are to be obtained and be reproducible. Such purity may be achieved by utilization of lead, tellurium, selenium and Isulphur which d0 not contain metallic impurities exceeding the order of 0.01%. Alternatively, starting constituents of lesser purity maybe utilized to `be of the 1. order of purity aforeindicated.k n Several impurities commonly found in commercial stocks` f of all yfour of `the constituents will reduce the Peltier ex-y hibited by compositions `,of the present kinvention'aud ing a deleterious eiect. n n n n Compositions'of lead and at least one of the group tellurium,` selenium, sulphur, .aforedescribed, aiording producedby the following method. The starting constituents, free of metallic `contaminants yas faforeindic'ated,

and preferably Vin a reduced state, arefmixed together,y yin the `proportions indicated hereinbefore and sealed in a tube or containerV preferably of 'quartzy or ,Vycor, the containery rst being evacuated. The V'tube and itsy conf f tents,are-then heated to the melting point of thek latter which occurs at a. temperature ranging fromr about y920," f

'C. for the terminal compositions graphically yillustrated f at ythe left-hand `extreme of Figure'S to 1085 C. kfor, the

terminal ycompositions illustrated at ythe yriglit-hand extreme ofFigure r3` and theleft-handfextreme,of Figure 4, respectively, `to 1115 C. rfor the terminal compositions illustrated (at the yright-hand yextreme ofy `Figure 4.

The particular temperature for any given ,composition illustratedr 'in Figures 3 andl 4 depends upon the meltf i f ing point of thatcompositionfwhich ycan, readily ducedy by those skilled inthe artffrom that of yrninal compositions given by Way of example. yDuring such heating the molten mass is ypreferably insure good mixing and then cooled. y ,y Aftery 'the composition has been formed as aforementioned, the 'solidified ingot can beremoved from the tube agitated to y `,and east in molds of graphite or the like under an, at y mosphe're of inert gas. More speciiically, kduring casting we have found it preferable to cover the mold with an inert gas such as, for example, argon or carbon dioxide under positive pressure. `This gas suppresses the vaporization rate of the molten composition, thereby reducing porosity of the casting.

The aforedescribed `alloying and casting steps should be carried out in crucibles which do not react with or contaminate the composition since, as aforeindicated, minor amounts of undesired impurities may very deleteriously aect the electrical and/or physical properties of the element. Suitable crucibles are, we have found, those made of carbon, Alundum, pre-fired lavite and Vycor or quartz. e

After casting the ingots `may be machined if necessary. The shaped ingots are then preferably annealed in a reducing atmosphere at from 540 C. to 815 C. for from 20 `to 10 hours. rIhis `annealing treatment insures homogeneity of 4the ingots ,and` enhances its electrial and physical properties. e

An alternate method of forming the composition comprises melting the constituents aforementioned in an open crucible under an atmosphere of argon or any inert and/reducing gas. Since the vapor pressure of selenium and sulphur are relatively high, some loss of selenium or sulphur may be encountered and adjustments in the initial amount of this constituent, Where used, must be made to account for this loss. In all other respects this method is `similar to that previously described.

Thermoelectric heat pump elements comprising alloys or compositions of the invention can be achieved as aforementioned through utilization of starting constituents o f such purity that the compositions and resultant alloy containsno more impurity than that aforeindicated.

Alternatively, an alloy of such purity may be afforded by forming the composition as to be hereinafter described from less pure starting constituents and reducing the impurity content by recrystallization from the melt. As-

cient to bring the resultant composition within the limits aforeindicated.

The aforementionedfalloys of lead and atleast oney of the group tellurium, selenium, sulphur, may be best described metallographically as two-phase-alloys. It has been observed that these two-phase alloys, when sectioned and examined microscopically, comprise a major phase comprising crystal grains varying usually'from 1 to millimeters in size and between such grains there exist thin, relatively darker regions of a second'phase. The grains of the primary phase are crystals of the intermetallic compounds lead-telluride, lead-selenide and leadsulphide (or mixed crystals thereof), which contain approximately 61.89%, 72.41% and 86.60% lead by weight, respectively. The darker second phase, clearly discernible at the grain boundaries, is lead containing a minor concentration of selenium, tellurium or sulphur. The function of the second phase in such alloys is thought to be three fold. First, the thermal equilibrium between the two phases, which is established by the heattreatment aforementioned, induces negative Peltier and conductivity in the primary lead-telluride, lead-selenide or lead-sulphide phase which, because of its high concentration in the alloy, controls the electrical properties of the two-phase alloy. Secondly, the thin layers act as a cementing agent for the grains of the primary phase, thereby improving the mechanical strength of the alloy when compared with that of the pure intermetallic compound. lThirdly, this cementing action of the second phase alfords good electrical conductivity in the polycrystalline .alloy by rendering the intergrannular component of electrical resistivity negligible. We have found that the actual concentration of second phase is not critical so long as the composition is maintained within the aforementioned specified ranges.

With regard to such aforementioned specified ranges for the various compositions aforedescribed, it will be observed that in each case there is an excess of lead over and above the amount thereof necessary for satisfying the stoichiometric proportions of the compound formed with the second constituent or constituents, i.e. the tellurium, selenium or sulphur. Taking by way of example the aforementioned terminal compositions, it will be noted that the first terminal composition consisting substantially of lead and tellurium contains from 61.95% to 65.0% by weight lead, or from .16% to 8.9% lead by Weight of the total'composition over-and above the,61.89% lead stoichiometrically necessary to" combine with the tellurium. Similarly, the terminal composition consisting substantially of lead and selenium contains from .15% to 10.4% lead by Weight of the total composition over and above the 72.41% by weight lead stoichiometrically necessary for combination with the selenium. The same is, of course, true Awith respect to the terminal composition consisting substantially of lead and sulphur wherein the amount of Vlead specified forcomposition of the present invention is from .23% to 4.7% lead by Weight of the total composition more than that necessary to stoichiometrically combine with the sulphur present, i.e. 86.60% by weight lead. Similarly, for any of the compositions intermediate the aforementioned zio iti

terminal compositions there exists th'eiciiisi tion ranges specified in Figures 3 and.4 anexcess of lead over andv above that stoichiometrically necessary to combine with the tellurium-selenium or selenium-sulphur constituent in percent by Weight varying according to the relationship of such intermediate composition to the terminal compositions. Y

The excess of lead aforementioned inherent in lall of the compositions aforedescribed within the ranges thereof graphically illustrated in Figures 3 and 4 may be denominated an impurity with respect to the primary leadtelluride, lead-selen/ide and lead-sulphide phases, as the case may be. Such impurities, however, must be 'distinguished vfrom the undesired impurities discussed hereinbefore in connection vwiththe required, purity'requirements of the compositions or alloys. kAccordingly, for purposes of thisv specication and the appended claims such excess lead amounts purposely present in the compositions will be denominated beneiicial impurities. As

aforeindicated, it is the presence of such beneficial impurities that affords the compositions of this invention their desired electrical characteristics and distinguishes the semi-metallic alloys or compositions of this invention from the primary phase intermetallic compounds leadtelluride, lead-selenide and lead-sulphide. More speciiically, such beneficial impurities afford such intermetallic compounds, whether the binary compounds of the terminal compositions or the ternary alloys of the intermediate compositions, a lower resistivity without proportional sacrice of Peltier E.M.F. or low thermal conductivity and which results in a composition having what may be termed semiconductor-like electrical conductance or conductivity. It should be noted that such excess lead vinduces in each ofvthe compositions aforedescribed a Peltier of negative polarity and negative conductivity, thereby affording a therrnoelectric heat pump element having utility in, for example, the single element heat pump of Figure 1 or in combination with a thermoelectric heat pump element of opposite polarity, the double or compound heat pump of Figure 2.

We have further discovered that the electrical characteristics desirable in thermoelectric elements for heat pump Yapplications of the aforedescribed lead-tellurium, lead-selenium, lead-tellurium-selenium, lead-sulphur and lead-selenium-sulphur compositions of the aforementioned range and purity (for convenience hereinafter denominated base compositions) can be markedly and advantageously altered in a reproducible manner by the addition thereto of controlled amounts of matter other than the constituents of the base composition. Such additions may also be denominated beneficial impurities as distinguished from undesired impurities as aforementioned. For convenience, these additions .are herein 4 designated promoters since, as will hereinafter appear,

they tend to enhance the electrical characteristics `desired in thermoelectric heat pump elements of the base composition. Since, as will hereinafter appear, the amounts of such promoters in terms of percent by Weight of the base composition are Very small, for such promoters to be most effective and afford reproducibility of electrical properties the base composition to which they are added` should be'of even greater purity, i.e. contain even less of undesired and uncontrolled impurities. As a practical matter we have found 'that the base composition should be of such purity in this regard as to contain not in excess of 0.001% by weight undesired impurity. Similarly,V

where such promoters are added, the lead content of the base composition should be slightly less; e.g. a maximum of 63.0%, 73.5% and 87.10% by weight, respectively, for the various terminal compositions.

As has previously been observed, all of the aforedescribed base compositions exhibit negative Peltier and negative conductivity. By the addition of the promoters to be hereinafter describedsuch negative properties may be enhanced by the addition of certain promoters while the polarity of the electrical properties of the base composition may be reversed by the addition of certain other promoters. Accordingly, certain promoters will be denominated positive promoters and certain others will be denominated negative promoters, as hereinafter derned, and the resultant alloy or composition may be positive or negative alloy or composition, as also hereinafter defined.

Negative compositions or alloys are to be understood throughout this specification and appended claims as meaning an alloy or composition which exhibits negative conductivity as evidenced by Hall etect measurements or thermoelectric effect measurements, both taken at room temperature. Similarly, positive `compositions or alloys are to be understood as meaning an alloy or composition which exhibits positive conductivity as evidenced by I -Iall effect measurements or thermoelectric etect measurements,both taken at room temperature.

Negative promoters are` those which when addedoto the base alloys aforedescribed alter the electrical conductivity thereof without changing the polarity of the conductivity or Peltier E.M.F. of the base alloys (it being negative according to the preceding definition). Positive promoters are those Which when added in amounts hereinafter disclosed to the base alloys aforedescribed and certain base alloys hereinafter disclosed are effective in enhancing the electrical conductivity in a positive sense as above defined with only slight descrease in the thermoelectric power of the promoted base alloy. Certain ofV such positive promoters when added to the base alloys aforedescribed cause at first with very small additions` reduction in the conductivity of the alloy to a minimum value beyond which further increase in the concentration of the positive promoters causes an increase in the conductivity of the alloy accompanied by a reversal in the polarity `of the conductivity and Peltier E.M.F., i.e. from negative to positive.

The functions of such negative and positive promoters should be contrasted for the sake of clarity as follows:

(l) Increasing concentrations of the negative prometers cause increases in the conductivity and `decrease of the Peltier of the resulting alloy as compared to that of the `base alloy while preserving the negative polarity of the conductivity and Peltier thereof.

(2) Increasing concentrations of the positive promoters cause initially reductions in the conductivity and increase in the Peltier of the base alloy until a minimum conductivity is reached whereupon the Peltier E M.F. and conductivity reverse polarity to the positive sense and further increase in the concentrations of the positive promoters causes increase in the conductivity and decrease in the Peltier in the resulting alloy.

The promoters which we have found effective for the purposes of the present invention when added in minor amounts to the base compositions aforementioned will for convenience be discussed in terms of the `terminal compositions modied as to purity and lead content as aforedescribed, it being understood that such promoters may also be added to any of the intermediate compositions with beneficial results. In this case the promoters added should be proportioned both in kind and in amountV according to the relative concentrations of the terminal compositions in the intermediate composition comprising a mixture of such terminal compositions.

Table I below., first column thereof, lists the additions which we have found effective as negative promoters when added to the aforementioned lead-tellurium base alloys or compositions. The second column of Table. I lists the order of the maximum concentration limits by weight percent of such promoters to the ybase alloys effective for achieving the objects of the invention. It is to be understood that these concentration limits are the maximum which effectively alter the electrical properties of the base alloy. Concentrations in excess of the stated amounts of such additives have no` appreciable effect in beneficially altering theielectrical properties with which this invention is concerned, and in this sense the limits indicated are to be considered critical. The third and fourth columns of Table I set forth the electrical properties at room temperature of lead-tellurium alloys promoted with the maximum useful concentrations of the negative promoters after high temperature annealing as hereinbefore described.

Table I Order of maximum Peltier Resiseiectlve E.M.F tivity, N egatlve promoters eoncentravolts, ohm-cm tion limits 0.022 0.00020 by weight percent, 0.80

Uranium .1 The range set forth is discussed below.

As previously mentioned, certain positive promoters may also be alloyed with the aforementioned lead-tellurium base alloys and such promoters are listed in column l of Table II. The second column of Table ll like the corresponding column of Table I sets forth the order of the maximum concentration limits by weight percent of such promoters to the base alloys effective for achieving the objects of the invention. Again it will be ob served that concentrations of the positive promoters to the lead-tellurium base alloys in amounts in excess of that contained in column 2 of Table Il have no appreciable effect in beneficially altering the electrical properties with which this invention is concerned, and in this sense the limits indicated are to be considered critical.

Column 3 of Table II sets forth the minimum concentration by weight percent of the positive promoters listed affording useful heat pump elements of positive polarity.

Columns 4 and 5 set forth the Peltier and resistivity characteristics at room temperature of the alloy or composition resulting from the addition of the aforedescribed positive promoters in the amount shown in column 2 after high temperature annealing as aforedescribed.

Table 1l Order of Order of minmaxmum imum etec- Positive effective tive concen- Peltier Resispromoters concentratration limit E.M.F tivity, tion limit by weight volts ohm-cm by weight percent percent 0. 00 0. 0000 +0. 052 0. 00074 0.10 0. 0009 +0. 059 0 00070 1 0. 25-1, 00 l 0. 01-0. 04 +0. 077 0. 0029 Arsenic 1007-025 10002-0006 +0081 0.0045

1 The rangeset forth is discussed below.

As aforementioned, the lead-tellurium base alloy previously described is a two-phase alloy. When the aforedescribed promoter additions are incorporated inthe base alloy, such promoter additions become distributed; be-

We have discovered that the the case of bismuth, thallium and arsenic the maximum effective concentration isV dependent upon the lead con-- tent of the lead-tellurium 'base alloy within the ranges stated therefor as, shown in-Figure 3. We have found 1.20% by weight bismuth to be the maximum effective mum eifective bismuth concentration is somewhat less,

that is ranges down to 0.60% by Weight when the lead content ranges down to 61.95 Similarly, in the case of thallium, the maximum effective concentration is dependent upon the lead content of the lead-tellurium base alloy within the range stated therefor. We have found 1.00% by weight thallium to be the maximum effective concentration for lead-tellu-rium base alloys containing 63.0% lead; for base alloys containing less lead the maximum effective thallium concentration is somewhat less, that is ranges down to 0.25% by Weight when the lead content ranges down to 61.95 Similarly, in the case of arsenic the maximum eiective concentration is dependent upon the lead content of the lead-tellurium base alloy within the range stated therefor and rangesfrom 0.25% for base alloys containing 63.0% lead down to 0.07% for base alloys containing 61.95% lead. As indicated in Table II, the minimum elfective concentration weight percent for heat pump elements in the case of,

thallium promoted base alloys ranges from 0.01% to 0.04% as the lead constituent of the lead-tellun'um base composition 'varies from 61.95% to 63.0%. Similarly, in the case of the arsenic promoted base alloy the minimum effective concentration weight percent-ranges from 0.002% to 0.006% as the lead content of the base alloy varies from 61.95% to 63.0%.'

This behavior of bismuth, thallium and arsenic is thought to be due to the formation of a bismuth-leadtellurium, thallium-lead-tellurium or an arsenic-leadtellurium complex Within the intergranular phase aforementioned which accounts for a portion of the addition. All other third element additions aforementioned, both positive andnegative, form complexes with the second or intergranular phase aforementioned to a much lesser extent than do bismuth, thallium and arsenic and for purposes of this invention, in the cases of such other additions these effects are inconsequential. Accordingly, no changes in the concentration limits thereof are necessary as the proportions of lead and tellurium in the base alloy vary within the range stated therefor.

In Tables I and II above, the Peltier and resistivity data given is in both cases 'for the *61.95%v lead,

balance substantially all tellurium composition contain-V ing'the promoter addition in question in the maximum effective amount indicatedV in the table (in the case of bismuth, thallium andy arsenic, the lower maximum eifective'amount indicated).

15 Gal Table III below, first column thereof, lists the additions whichwe have found effective as negative promoters when added tothe aforementioned lead-'selenium base alloys Vor compositions. The second column of Table III lists the order yof the maximumconcentration limits by weight percent of such promoters to the base alloys effective forachievin-g the objects of the invention. .It is y to Vb'e understood that theseconcentration limits are the maximum ,which Aeifectively'allter the electrical properties of the base alloy. Concentrations in excess of the stated amounts of such additives-have no appreciable effect in beneiiciallyaltering the electrical properties with which this inventionis concerned, 'and in this sense the limits indicated are to be considered critical. The third and fourth columns of Table III .set forth the electrical prop-- ertiesat room temperature of lead-selenium alloys promoted with the" maximum useful concentrations of theV negative promoters after hi'gh temperature annealing as hereinbfore described.

Table Ill Order of maximum Resistivity, F.,. ohm-cm.

effective concentration limits by weight percent Negative promoters .A ntimony Columbium l The range set forth is discussed below. v

LAS previously mentioned, certain positive promoters may' also be alloyed with the aforementioned leadselenium base alloys and such promoters are listed in column 1 of Table IV. The second column of Table lV like the corresponding column of Table III sets forth the order of the maximum concentration limits by weight percent of such promoters to the base alloys effective for achieving the objects of the invention. Again it will be lobserved that concentrations of the positive promoters to the lead-selenium base alloys in amounts in excess of that contained incolumn 2 of Table IV have no appreciable effect in benecially altering the electrical properties with which this invention is concerned, and in this sense the limits indicated are to be considered critical.

Column 3 of Table IV sets forth the minimum concentration eby weight percent of the positive promoters listed affording useful heat pump elements of positive polarity.

Columns 4 and 5 set forth the Peltier and resisi tivity characteristics at room temperature of the alloy or composition resulting from the addition of the aforedescribed positive promoters in the amount shown in column 2 after high temperature annealing as aforedescribed.

Tgzble IV Order of maximum effective concentral tion limit by weight percent O rder of minimum effective concentration limit by Weight percent Peltier E.M .F., volts Resistlvity,

Positive promoters ohm-cm.

Sodium Potassium As aforementioned, the lead-selenium base alloypreviously described is a two-phase alloy. When theaforedescribed promoter additions areY incorporated in theY base alloy, such promoter additions become distributed between the vtwo phases. Wel have discovered. thatl the nature of such distribution hasrnegligible effect onl the electrical properties of the composition in allY cases exl lead content ofv theV lead-selenium basealloy WithinA the range stated therefor. We have found 1.5% by weight antimony to be the maximum effective concentration for lead-selenium base alloys containing 73.5% lead; for base alloys containing less lead `the maximum effective antimony concentration is somewhat less, that is ranges dow-n to 0.20% by weight when the lead content ranges down to 72.45%.

This behavior of bismuth `and antimony is thought to be due to the formation of a bismuth-lead-selenium or an antimony-lead-selenium complex within the intergranular phase aforementioned which accounts for a portion of the addition. All other third element additions aforementioned, both positive and negative, form complexes with the second or intergranular phase aforementioned to a much lesser extent than do bismuth and antimony and for purposes of this invention, in the cases of such other additions these effects are inconsequential. Accordingly, no changes in the concentration limits thereof are necessary as the proportions of lead and selenium in the base alloy vary within the range stated therefor.

In Tables III and IV` above, the Peltier and resistivity data given is in both cases for the 72.45% lead, balance substantially all selenium composition containing the promoter addition in question in the maximum effective amount indicated in the table (in the case of bismuth and antimony, the lower maximum effective amount indicated).

Table V below, first column thereof, lists the additions which we have found effective as negative promoters when added to the aforementioned lead-sulphurV base alloys or compositions. The second column of Table V lists the order of the maximum concentration limits by weight percent of such promoters to the base alloys effective for achieving the objects of the invention. It is to be understood that these concentration limits are the maximum which effectively alter the electrical properties of the base alloy. Concentrations in excess of the stated amounts of such additives have no appreciable effect in beneficially altering the electrical properties with which this invention is concerned, and in this sense the limits indicated are to be considered critical. The third and fourth columns of Table V set forth the electrical properties at roomtemperature of lead-sulphur alloys promoted with the maximum useful concentrations of the negative promoters after high temperature annealing as hereinbefore described.

Table V Order of maximum effective Peltier Resistivity, Negative promoters concentration E.M.F., ohm-cm.

limits by volts weight percent Zirconium f 0. 40 0. 011 0. 00022 Indium 0. 50 0. 016 0. 00024 Blomine. 0. 35 0. 016 0. 00024 Chlorine... 0. l 0. 010 0. 00025 Titanium" 0. 20 0. 020 0. 00029 Iodine- 0.55 0.030 0.00042 Tantalum 0. 70 0.030 0. 00042 Bismuth. 11. 0-3. 0 0.035 0.00061 Antlmon 1 0. 50-3. 0 0.038 0100064 Gallium., 0. 30 0. 035 0. 00058 Columbium, 0. 40 0.032 0.00023 Uranium L 1. 0 0. 030 0. 00024 l The range sct forth is discussed below.

As previously mentioned, certain positive promoters may also be alloyed with the aforementioned lead-sulphur base alloys and such promoters are listed in column 1 of `T able VI.

The second column ofTable VI like the cially altering the electrical properties with which thisv described positive promoter in the amount shown in column 2 after high temperature annealing as aforedescribed.

Table V1 Order of Order of maximum minimum effective effective Peltier Resistivity, Positive promoters eonoentraconcentra- E.M.F., ohm-em.

tion limit tion limit volts by weight; by weight percent percent Silver 2.0 0.70 0.081 0.020

As aforementioned, the lead-sulphur base alloy previously described is a two-phase alloy. When the aforedescribed promoter additions are incorporated in the base alloy, such promoter additions become distributed between the two phases. We have discovered that the nature of such distributionhas negligible effect on the electrical properties of the` composition in all cases except that of bismuth and antimony. Accordingly, in the case of bismuth and `antimony the maximum effective concentration is dependent upon the lead content of the `lead-sulphur base alloy within the ranges stated therefor as shown in Figure 3. We have found 3.0% by` weight bismuth to be the maximum effective concentration for lead-sulphur base alloys containing 87.10% lead; for base alloys containing less lead the maximum effective bismuth concentration is somewhat less, that is ranges `downto 1.0% by weight when the lead content ranges down to 86.63 Similarly, in the case of antimony, the maximum effective concentration is dependent upon the lead content of the leadsulphur base alloy within the range stated therefor. We have found 3.0% by weight antimony to be the maximum effective concentration for lead-sulphur base yalloys containing 87.10% lead; for base alloys containing less lead the maximum effective antimony concentration is somewhat less, that is ranges down to 0.50% by weight when the lead content ranges down to 86.63%.

`This`behavior of bismuth and antimony is thought to be due to the formation of a bismuth-lead-sulphur or an antimony-lead-sulpuhr complex Within the intergranular phase aforementioned which accounts for a portion of the addition. All other third element additions aforementioned, both positive and negative, form complexes with the second or intergranular phase aforementioned to a much lesser extent than do bismuth` and antimony and for purposes of this invention, in the cases of such other additionsV `these effects are inconsequential. Accordngly, no changes in the concentration limits thereof `are necessary as theproportions of lead and sulphur in the base alloy vary within the range stated therefor.

In Tables V and VI above, the Peltier andresistivity data given is in both cases for 86.63% lead,

balance substantially all sulphur compositionlcontain-A i ing the Apromoter addition in question in the maximum effective amount indicated in the table'(in the case of bismuth and antimony,` the lower vmaximum effective amount indicated).` Y r i From the foregoing description, it will be noted that the maximum concentrations of beneficial impurityfor promoted terminal compositions vary from 4.22%` by weight (i.e. 3.02% excess lead plus 1.20% maximum `for any promoter) in the case of the terminal lead-tellurium composition, to 6.6% by weight (i.e. 4.1% excess lead t plus 2.5% maximum for any promoter) in the case of Aiv vweight (ie. 3.9% excess lead plus 3.0% maximum for any promoter); in the case of the terminal lead-sulphur composition. Thus, the maximum amount of benelcial impurity for any promoted composition is 6.9% by weight. While, as aforedescribed, the maximum amount of beneficial impurity for any unpromoted composition is 10.4% by weight. Accordingly, it can bei said that the maximum amount of Vbenecialimpurity for any composition aforede'scrbed is, in round numbers, about 10% by Weight.

Typical data ofthe aforedescribedv compositions are, by way of example, shown in Figures and 5a wherein the Peltier E.M.'F. in volts (left-hand scale) and p in watts per centimeter (righthand scale) are plotted against the log of the resistivity inclini-centimeter.

In Figure 5 curves G, H and I represent compositions consisting essentially of leadand tellurium, lead'and selenium, and lead -and sulphur, respectively, within the ranges aforeindicated, and which curves graphically illustrate variation in the relationship between Peltier coel`n`cient (1r) and resistivity (p) with additions of negative promoters to the respective base compositions (the right-hand ends of such curves representing the base compositions). Similarly, curves I, K and L represent compositions consisting essentially of lead and tellurium, lead and selenium, and lead and sulphur, re spectively, within the ranges afor'eindicated, and which curves graphically illustratevvariations in the value of the quotient Y k P with additions of negative promoters to the base composition (the right-hand ends of such curves representing the base compositions).

In Figure 5a .curves M, N and O represent compositions consisting essentially of lead and tellurium, lead and selenium, and lead and sulphur, respectively, within the ranges aforeindicated, and which curves graphically illustrate variation in the relationshipV between Peltier coeiicient (1r) and resistivity (p) with additions of positive promoters to the respective base compositions v(the right-hand ends of such curves representing the minimum effective concentration of positivenprornoters aforedescribed). Similarly, Acurves P, Q and R lrepresent compositions consisting essentially of lead and tellurium, lead and selenium, and lead and sulphur, respectively, within the ranges aforeindicated, and which curves 'graphically illustrate variation in the value of the quotient P Y with additions of positive"l promoters to theY baseconiposition (the right-hand ends of such curves represent ing the minimum effective concentration of positive promoters aforedescribed). l Y V AIn Figures `5 and 5u curves I, K, L and P exhibit maximum values of the quotient The curves Q and R exhibit extremal valuesfor the maximum concentration of positive promoter. The resistivity of the right-hand coordinate of Figures 5 and 5a (0.1` ohm-cm.) is characteristic of semi-conductor vmaterials. The resis'itivity of the left-hand coordinate of Fig ure's 5' and 5d (0;0001 ohmlcm.) is characteristio of a metal. The maximum and e'xtremal values aforementioned occur at values o'f Peltier coeicient and resistivity intermediate to those of'me'tals'and semi-conductors. Morei8 overas aforementioned, semi-metallic lements embodying the present invention exhibit low thermal conductivity approaching that of a semi-conductor, High values of the quotient i l and simultaneously the low values of thermal conductivity (K) afford the high performance factors desired for heat pump elements. l

Further illustrative datafor some of the compositions aforedescribed are'y shown byway of exampleeinFigure 6 wherein the left-hand scale is in terms of theperformance' co'eiiicient (refrigeration) anni@ Qb'l-.Qa .Y The right-hand yscale is in terms of Qa v(the pumping rate) in watts, both plotted against the horizontal scale of current density,ampp/c`m.2. In Figuren curve yS isla representative performance coein'cient curve for' compost tions consisting essentially of lead and tellurium while curve T isa representative curve ofthe vpumping lrate (Qa) for the sarne compositions: Similarly', curve Ui is a representative performance coeicieiit curve for compositions consis'ting'essentially of leadY and sulphur, while curveV is a representative' pumping rate curve (Qa) for the same compositions. From Figures and 5u it will be readily ascertained that representa-tive curves for cornpositions consisting essentially of` lead Vand selenium will fall somewhat intermediate the vcurvesr in Figure 6 for the lead and tellurium'and lead and sulphur compositions,-

respectively; I

As will be apparent to those skilled in the art, all of the semi-'metallic compositions aforedescribed exemplary of the thermoelectric heat pump elements comprising binary metallic compounds and ternary comp'ositioiisr or alloys thereof having semiconductor-'like conductivity afford heat. pump characteristics of sullicent magnitude for practical application. By way vof example, data is given in Table VII below illustrating siich improved heat pumping characteristics. p

In Table VII the irst column in terms of composition and percent by weight lists various examples of compositions of the class described; theseond cluinn indricates the Peltier' E.M.F.in volts derivable fromfch of such compositions; theV third column lists foreach' of such compositions the thermal co'ridctivity in lwatts" per centimeter degrees Centigrade; the fourth column lists for each of such compositions their resistivityrinmohmf cm.; and the fifth 'colii indicates for each of such compositions the performance factor thereof. It will be noted `that `tli'e ex'ar'iplesl f compositions" given include examples of Ieach of the Base alloys and at least one example ofeach of the base alloys to which a promoter has been added.

Tabley VII oomposmonnreent by E M.F., Y may,

. weight volts Electrical ano 0.00`i0f `l 0.0130 Y 4-0. oss' Referring to Table VII, a comparison of the performuance factor column indicates that the promoted alloys Y Table VIII Performanco Heat Performfactor Current Temp. pump ance Composition (caleudensity, dii., rate, coefficient j lated, amp/cm.2 C. watts (refriger- 1r7/o2Kp) atlll) .62% Pb 38,7 Te 1.58 1.7 0.002 5.

i(negative polar- 63 g g5 m' 12. 0 9. 7 0. 03V 0. 42

62% Pb 38% Te 1. 5s 1.3 0.0007 14. 3

+0.115% :si (nega-` so '-'g g- Ve polarm 122s sli 0075 1'3 62% Pb 38% Te 1. 58 1.a 0. 0i 17. 0

+0,10% Bi (negas1. gg3g me polam' 1210 71s 0I 10 2' 1 The data of Table VIII shows that the perfomance coefficient and heat pumping rate increase as the bismuth concentration is increased. Similar effect can be observed in connection with other of the base alloys and with other promoters of the given base alloys. It will be noted also thatfor a given composition and current density, the induced temperature difference and heat pumping rate are related. As one is increased the other is decreased. In the range shown, the induced temperature difference and heat pumping rate increase with increased current density while the performance coefficient decreases. Since it is usually desirable to have most of these parameters as high as possible, there is no particular optimum current density; instead the current density is a parameter to be chosen according to the application involved.

Table VIII above` gives typical data for all of the aforedescribed compositions applicable Vequally to thermoelectric heat pump elements of positive as well as negative polarity. ly way of example Table IX following gives heat pumping data for a specific thermoelectric heat pump element of positive polarity.

Table IX Current T Heat Performein Composition density, dirti) giit amp/cm.2 O. watts (refrigeration) 3. l5 l. 3 0. 015 3 7 62% Pb-ss7 're+0.2 'ri Y (positive poolarlty). gi 25. 2 4. 8 122 0. 5

With a single element heat pump of the type shown schematically in Figure l atemperature suppression at the cooling junction of 25 C. has been attained, and a temperature elevation at the heating junction of at least' 100 C. has been attained.` Similarly,V as indicated in Tables VIII and IX, a performance coefficient of cooling as high as 17 can be attained. In fact,` referring to Figure 6, 1t will be observed that as the current density approaches smaller values the performance coefficient rises. steeply indicating that even higher performance coefiicients` can`be attained at lower current densities. As aforementioned, similar performance coefficients can` be attained at the heating junction since the performance coefi'icient for heating is the performance coecient for coolingplus one. Moreover, as will also be apparent from Figure 6, pumping rates approaching .2 of a watt for a singleV unit of a quarter inch diameter at a current density in excess of 25 amperes per square centimeter are attainable. Since the pumping rate increases as the performance coefficient decreases the current density for a practical applicationfshould be chosen to give simultaneously the highest possible values of performance coefficient and pumping rate according to the nature of the application. p Y

We have further found that suitable additional positive thermoelectric elements exhibiting positive electrical characteristics as previously defined, and useful for heat pumping may comprise lead and tellurium in which there is an excess of tellurium over and above the amount thereof necessary for satisfying the stoichiometric proportions of the compound` lead-telluride. Such additional alloys or compositions may contain added beneficial impurity substances, and such substances, as before, are also denominated hereinafter as positive promoters. These additional alloys or, compositions may also be characterized as binary metallic compounds of slightly imperfect composition by reason of the excess of tellurium over lead and/or containing added beneficial impurity substances. Such additional alloys or compositions, as previously related, may be termed semi-metallic alloys or compositions, and have semiconductor-like conductance, (both electrical and thermal), similarly to the binary and ternary alloys already discussed.

More specifically, we have found especially useful for thermoelectric elements exhibiting positive electrical characteristics, base alloys .or compositions consisting essentially of lead and tellurium in which lead is present inthe range of from 58.0% to 61.8% by weight and the balance in the range of from 42.0% to 38.2% by weight tellurium.

The portions of the lead and tellurium constituents as last set forth must be considered critical if the compositions are to have the electrical properties desired in heat pumps.

Asbefore, not only is the range last described of the limit of tolerable metallic impurity in the nal composition has been found to be on the order of 0.01% and the `composition must be substantially oxygen free if the mechanical and electrical propertiesdesired are to be obtained and be reproducible. Such purity may be achieved in the manner already described in the fabrication of the aforementioned lead and tellurium, selenium and sulphur compositionscontaining an excess of lead. An essential difference in the method of making the alloys of the last noted tellurium rich lead-tellurium compositions resides in the rate of cooling after annealing. In this regard, we have found a cooling rate of C. per hour to be sufficiently low to provide the element with adequate mechanical strength.

The tellurium rich base lead-tellurium alloys, as in the case of the first described alloys, may also be described metallographically as a two-phase alloy. The excessv of tellurium in the tellurium rich compositions behaves much as the excess of` lead in the first discussed alloys in that it controls the polarity of `the conductivity and thermoelectric power inthe primaryjlead-telluride phase, and it also forms a detectable second phase at the grain boundaries as already above discussed. The compositions are further adequately stable for temperature applications ranging up to 400 C.

It is to be observed from the foregoing that the "tellurium rich base lead-tellurium compositions contain from 38.20% to 42.0% by` weight tellurium, or from 0.14% to 6.7% by weight tellurium of the total composition over and abovethe tellurium stoichiometrically necessary to combine with the lead.

.The excesses of tellurium aforementioned in the compositions last described may be denominated impurities with respect to the primary lead-telluride phase. Such impurities, as before, must-.bedistinguished-from any undesired impurities as previously discussed, Accordingly, and as before, for purposes of this specification and the appended claims, such excess tellurium amounts `purposely present in the composition are denominated beneficial impurities. Likewise, it is the presence of such beneficial impurities that affords these last, mentioned additional compositions their desired electrical lcharacteristics andv distinguish such semi-metallic alloys or compositions of the inventionl from the primary phase intermetallic lcompound lead-telluride. Such beneficial impurities afford thev intermetallic compounds lead-telluride a lower resistivity without proportional sacrifice of Peltier or low thermal` conductivity and results in ak composition having what may be termed semiconductor-like electrical conductance or conductivity. It should be notedthat such excess tellurium induces inthe compositions.-aforedescribed a- Peltier of positive polarity and positive conductivity, thereby affording a thermoelectric heat pump element havingl utility in the manner already described.

We have further discovered that the electrical characteristics of thermoelectric elementsffor heat pumps of the last v'mentioned range of excess tellurium impurity (again for convenience hereinafter denominated base compositions) can be markedly 'and advantageously altered in a reproducible manner by the addition thereto of controlled amounts of matter other than the constituentsl of such base compositions. Such additions are also here denominated beneficial impurities as distinguished from undesired impurities as previously discussed. Again, such additions are denominated promoters since, as will hereinafter appear, they tend to enhance the electrical characteristics desired in thermoelectricl heat pump elements of the last mentioned base composition. As before, the amounts of such promoters in tenns of percent by weight of the base composition are very small, and for such promoters to be most effective and afford reproducibility of electrical properties the base composition to which they are added should be of even greater purity, i.e. containing even less of undesired and uncontrolled impurities. As a practical matter, we have found that the tellurium rich base leadtellurium compositions should be of such purity in this regard as to contain not in excess of 0.001% by weight undesired impurity. Similarly to the first disclosed promotedv compositions the lead content of the instant vbase composition should be in the range of from 59.0% to 61.8%*lead by weight as against the range of lead of from 58.0% to 61.8% by weight above disclosed for the unpromoted compositions.

As already observed, the aforementioned base compositions of tellurium rich lead-tellurium compositions exhibit positive Peltier and positive conductivity. By the addition of the promoters to be hereinafter described, such positive electrical properties may be enhanced by the addition of certainV promoters and such positive promoters in connection withA the last mentioned basealloy effect an increase in the conductivity of the alloy andslight decrease inthe Peltier Table X 22 properties atA room temperature of theaforementioned tellurium rich lead-tellurium base'compositions` promoted withA the usefulconcentrations of the' positive promotersfafter annealing.

From the foregoing description it will be noted that the' maximum concentrations of lbeneficial* impurity for the last disclosed promoted tellurium rich lead-tellurium compositions varies from aminimum of'0.28% by weight (i.e., 0.14% by weight excess tellurium plus 02I4% by weight sodium) to 5.5% by weight (iie. 4.9%Y by' weight Y excess tellurium plus 0.60%V by weight thallium'), which fall Well withinY the aforementionedv maximum amount of beneficial impurity of about 10% by weight. u

Typical dataof the last aforedescribed compositions s shovv'n by way of example in Figure 7, which. is similar to Figues 5 and 5u, wherein the Peltier E.M.F. in volts (left-hand scale) and lrz i in watts per centimeter (right-hand: scale)- are plotted below, first column thereof, lists the additions which we i have found effective as positive promoters when added to the last mentioned tellurium rich lead-tellurium base compositions. The second column of` Table X lists the order of the maximum effective concentration. limits by weight percent of such promoters to the base alloy effective for achieving the objects of the invention. It is understood that these concentration limits are the maximum which effectively alter the electricalproperties of the notedV base alloy; Concentra-tions in excess ofthe stated amountsv of such additives have no appreciable eEect in beneficially altering the electrical properties with .which this invention is concerned, and in this sense the limits indicated are to be considered critical. The third and fourth columns of Table X set forth the electrical against the resistivity in ohm-centimeter. Y v

In Figure 7, curve X represents compositions of tellurim rich leadLtelluriuIn Within the range afore-indicated and which curve graphically` illustrates variations in the' relationship between Peltier coefiicient (1r) and resistivityv with addition ofA positive promoters tothe last noted base compositions (the right-hand end of such curve representing the effective concentration of positive promoters last described), Similarly, curve W represents the; last aforementioned tellurium richtle'adtellurium compositions Within4 the range .afore-indicated and Whichl curve graphically illustrates variations in the value of the quotient p The resistivity of the right-hand coordinate of Figure 7 (0;1 ohmcmr.) is characteristic ofsemic'onductor materials. The resistivity of the left-hand coordinate of Fig urev7 (0.00041l ohm-cm.) is characteristic of a metal. The maximum-.and extrenial values aforementioned occur at values of Pelti'er'co'eflicient' and resistivity intermediate to those ofmetals and' semiconductors;- Moreover, aspre'- viously' mentioned, semi-'metallic elements Vembodying thel present invention exhibit'low tl'i'ernial'V conductivity approaching that' offa semiconductor. High' values of the quotientA i Y and: simintan'eosly the low values of'thennal' conductivity (K) afford the high performance effect desired for heat pump elements.

, In Table XI below, the first column in terms of composition and percent by weight, lists various examples of compositions of the class last described; the second column indicates the Peltier in volts derivable from each of such compositions; the third column lists for each yof such compositions the thermal conductivity in watts per centimeter degrees centigrade; the fourth column lists for each of such compositions the resistivity in ohm-cm.; and the fth column indicates for each of such compositions the performance factor thereof. It will be noted that the examples of compositions given include tellurium rich lead-tellurium base alloys and including each of the positive promoters last discussed.

Weclaim:

1. A thermoelectric heat pump comprising, at least a pair of thermoelectrically dissimilar electrical conductors joined in circuit to provide at least a pair ofthermoelectric junctions, at least one of said conductors being a semimetallic element selected from the class of binary and ternary alloys of slightly imperfect stoichiometric composition and having semiconductor-like conductivity, said l semi-metallic element affording transfer of heat therethrough when said circuit is energized, thereby effecting cooling at one of said junctions and heating at the other of said junctions.

2. A thermoelectric heat pump according to claim 1 in which the imperfection of composition of said semi-metallic element comprises a departure from stoichiometry of said alloy by an amount not in excess of 10.4` percent y by weight of said alloy.

3. A thermoelectric heat pump according to claim 1 in which the imperfection of composition of said semi-metallc element comprises a stoichiometric excess of one of the constituents.

4. A cold producing thermoelement comprising two circuit members of diierent respective materials, a heat absorbing element having good heat conductivity and slight thermoelectric power conductively joined intermediate said members to form together therewith a thermoelectric junction, at least one of said two members consisting of a binary compound of two metals of a slightly imperfect composition departing from perfect stoichiometry by an amount of at most 2 percent by weight of the total material of said member, and having semiconductorlike electric conductance.

S. In a thermoelement according to claim 4, said `amount consisting of an excess of one of the two metals over the other.

6. A cold producing thermoelement, comprising two circuit members of different metallic materials, heat absorbing element having good heat conductivity andrslight .thermoelectric power of large diierential thermoelectric power, and an intermediate part of larger conductance and of negligible differential thermoelectric power as compared with said two members, said members and said part being joined together to form a thermoelectricjunction, at least one of said members consisting of a binary compound of two metals of a slightly imperfect composition departing from perfect stoichiometry by an amount of at most 2 percent by weight of the total material ofsaid member and having semiconductor-like electric conductance.

` 7. A cold producing thermoelement, comprising two circuit members of different metallic materials, and an intermediate part of larger conductance and of negligible differential thermoelectric power as compared with said two members, said members and said part being joined together to form a thermoelectric junction, at least one of said members consisting of a binary compound of two metals of a slightly imperfect composition departing from perfect stoichiometry by an amount of at most 2 percent by weight of the total material of said member and having semiconductor-like electrical conductance.

8. A thermoelectric heat pump comprising a pair of circuit members joined in circuit to provide a thermoelectric junction, at least one of said two members consisting of abinary compound of two metals in which there is a departure from perfect stoichiometry by an amount not in excess of 10.4% by weight of said one member and having semiconductor-like electric conductance.

9. A thermoelectric heat pump comprising a pair of circuit members joined in circuit to provide a thermoelectric junction, at least one of said two members consisting of a binary compound of two metals in which there is a departure from perfect stoichiometry consisting of an excess of one of the two metals over the other by an amount not in excess of 10.4% by weight of said one member and having semiconductor-like conductance.

l0. A thermoelectric heat pump comprising a pair of circuit members joined in circuit to provide a thermoelectric junction, at least one of said two members consisting of a binary compound of two elements in which there is a departure from perfect stoichiometry consisting of a stoichiometric excess of one of ysaid elements over the other of said elements and a beneficial impurity substance not in excess of 6.9% by weight of said one y member and having semiconductor-like conductance.

11. A thermoelectricheat pump comprising a pair of circuit members joined in circuit to provide a thermoelectric junction, at least one of said two members being a semi-metallic element from the class of binary and ternary alloys of metals in which there is a departure from perfect stoichiometry by an amount not in excess of 10.4% by weight of said one member and having semiconductor-like conductance.

12. A thermoelectric heat pump comprising a pair of circuit members joined in circuit to provide a thermoelectric junction, at least one of said two members being a semi-metallic element from the class of binary and ternary alloys of metals of imperfect composition in which there is a departure from perfect stoichiometry consisting of an excess of one of the metals of an amount not in excess of 10.4% by weight of said one member and having semiconductor-like conductance.

13. A thermoelectric heat pump comprising a pair of circuit members joined in circuit to provide a thermoelectric junction, at least one of said two members being a semi-metallic composition from the class of binary and ternary alloys of elements of imperfect composition in which there is a departure from perfect stoichiometry consisting of a stoichiometric excess of at least one of said elements and a beneficial impurity substance not in excess of 6.9% by weight of said one member and having semiconductor-like conductance.

References Cited in the tile of this patent UNITED STATES PATENTS UNITED STATES PATENT .oEETcE CERTIFICATE CORRECTION Patent No. 2,896,005 July 21', 1959 Robert W., Fritts et al.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.-

Column line 42, for ."sulphur of read sulphur or column 8, lfline 59, for "electrial" read electrical line 64, for "and/reducing" read and/or reducing column l2, Table I, heading to second column, last line thereof, for "percent, 0.80'. read percent same table, same second. column, last line opposite "Uraniumu Vstrike out the leaders and ins ert. instead 0.80 third column, heading thereto, for "PeIti-er. E.M.F. volts, 0.022" read Peltier EJVLF. volts same column, last lirie, strike 'outthfe leaders and insert instead -0.022 same table I, fourth column, heading thereto, for "Resistivity, ohm-cm., 000020'l read Resistivity, ohm-cm. same table I, same fourth column, last line, strike out the leaders and insert instead 0.00020 columnlS, line 75, for :"hfereinbfore" read hereinbefore Signed and sealed this 21st day of June 1960.

(SEAL) Attest:

KARL H. AXLINE Attes ting Officer ROBERT C. WATSON Commissioner of Patent;

UNITED STATES PATENT orricE CERTIFICATE OF CORRECTION Patent No. 2,896,005 July 2l', 1959 Robert W., Fritts et al.

It is hereby certified that error appears in the printed specification of' the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 42, for "sulphur of" read sulphur or column 8, *line 59, for "electrial" read electrical line 64, for "and/reducing" read and/or reducing column l2, Table l, heading to second column, last line thereof, for "percent, 0.80K read percent same table, same second column, last line opposite "Uranium' Ystrike out the leaders and insert.` ins tead 0.80 third column, heading thereto, for "PeItLzer-- E.M.F. volts, 0.022" read Peltier E.IVI.F volts same column, last line, strike 'outthfe leaders and insert instead -0.022 same table l, fourth column, heading thereto, for "Resistivity, ohm-cm., 0.00020" read Resistivity, ohm-cm. same table I, same fourth column, last line, strike out the leaders and insert instead 0.00020 columnlS, line 75, for "hereinbfone" read` hereinbefore signed ana sealed this 21st day 0f- June i960.

(SEAL) Attest: KARL H. AXLINE ROBERT Co WATSON Attes ting Officer Commissioner of Patent: 

4. A COLD PRODUCING THERMOELEMENT COMPRISONG TWO CIRCUIT MEMBERS OF DIFFERENT RESPECTIVE MATERIALS, A HEAT ABSORBING ELEMENTS HAVING GOOD HEAT CONDUCTIVITY AND SLIGHT THERMOELETRIC POWERS CONDUCTIVELY JOINED INTERMEDIATE SAID MEMBERS TO FORM TOGETHER THERWITH A THERMOELECTRIC JUNCTION, AT LEAST ONE OF SAID TWO MEMBERS CONSISTING OF A BINARY COMPOUND OF TWO MATALS OF A SLIGHTLY IMPERFECT COMPOSITION DEPARTING FROM PERFECT STOICHIOMETRY BY AN AMOUNT OF AT MOST 2 PERCENT BY WEIGHT OF THE TOTAL MATERIAL OF SAID MEMBER, AND HAVING SEMICONDUCTORLIKE ELECTRIC CONDUCTANCE. 